1. Field of the Invention
This invention relates generally to an improved stereolithographic method and apparatus of the type for building a part at the surface of a volume of liquid resin, the improvement comprising building the part on the surface of a layer of the resin, which layer is supported by a volume of a dense, immiscible liquid, and then performing additional stereolithographic processing on at least a portion of the built part while it is immersed in the liquid.
2. Cross-Reference to Related Applications
This application is related to the following U.S. patent applications all of which are incorporated herein by the following reference:
______________________________________ Serial No. Status ______________________________________ 07/182,830 U.S. Pat. No. 5,059,359 07/183,015 U.S. Pat. No. 5,015,424 07/182,801 U.S. Pat. No. 4,999,143 07/183,016 U.S. Pat. No. 4,996,010 07/268,816 U.S. Pat. No. 5,058,988 07/268,907 U.S. Pat. No. 5,059,021 07/268,408 Abandoned 07/268,428 Abandoned 07/268,429 Pending 07/331,644 Pending ______________________________________
3. Background of the Invention
Stereolithography refers generally to the process of reproducing a part by curing successive two-dimensional cross-sections of the part at the surface of a volume of a liquid resin and then stacking the cross-sections together to form a high resolution reproduction of the part. The first step in the process is the part building step, whereby successive two-dimensional cross-sections of the part are formed using an apparatus known as a stereolithography apparatus (hereinafter "SLA"). The SLA builds the part by tracing successive cross-sections of the part at the surface of the liquid resin with synergistic stimulation, such as a UV laser beam or the like, at a sufficient exposure so that the exposed resin hardens, and successively lowering the cured cross-sections below the surface so that fresh liquid resin will flow over the cured cross-sections which will form the next cross-section. The extent to which the resin hardens beyond the surface is known as the cure depth. The SLA lowers the cured cross-sections to within a layer thickness of the surface of the liquid resin so that the next-cured cross-section will adhere to the already cured cross-sections. In this manner, a laminar build-up of the part is formed at the surface. This process is described in more detail in U.S. Pat. No. 4,575,330 (hereinafter the '330 patent), which is hereby fully incorporated by reference herein as though set forth in full.
In one embodiment of an SLA, illustrated in FIG. 1a, a volume of UV curable liquid resin 22 or the like is placed in vat 21 to form a designated working surface 23. UV light source 26 or the like is placed above the surface to produce synergistic stimulation which strikes the surface at point 27. The light source is controllably directed by computer control system 28 to draw successive patterns representing successive two-dimensional cross-sections of a part on the surface 23.
Cured cross-sections are formed on platform 29 which is also under the control of computer control system 28. After a cross-section is formed, the computer directs platform 29 to lower itself a particular distance further into the liquid resin 22, so that the cured cross-sections will be lowered below the surface, allowing fresh liquid resin to flow over the cured cross-sections. Optionally, the platform is lowered by more than one layer thickness, and then raised to within one layer thickness after the fresh liquid resin has flowed over the surface. Then, the computer directs the light source to draw out the next cross-section on the surface at a particular exposure so that the cross-section being formed has a sufficient cure depth to penetrate beyond the surface of the resin and adhere to the cured cross-sections.
In FIG. 1a, the laminar build-up of the part is indicated by reference numeral 30, and successive cross-sections of the build-up are indicated by reference numerals 30a, 30b, and 30c, respectively.
Another embodiment is illustrated in FIG. 1b. As illustrated, a layer 22 of UV curable liquid resin or the like is supported by a heavier, UV transparent, non-miscible liquid 32. In addition, the non-miscible liquid supporting the layer of liquid resin is placed in a vat 21, which has a UV transparent window 33, constructed of quartz or the like, at the bottom of the vat. UV light source 26 or the like is placed below the bottom of the vat and synergistic stimulation from the source is directed by the control computer to pass through the window and the intermediate liquid to strike and cure the liquid resin from below, which curing takes place at the interface 23 between the resin and the non-miscible liquid, which interface is the lower surface of the resin layer. The UV light strikes the resin/liquid interface at point 27, and is directed by the computer to draw patterns representing cross-sections at the interface.
The first cured cross-section adheres to platform 29. In certain instances, other cross-sections may also adhere to the platform. After a cross-section is formed, platform 29 is controllably directed by the control computer to raise itself a particular distance so that a corresponding volume of fresh liquid resin will flow under the cured cross-sections at the interface 23. Optionally, the elevator may be raised more than one layer, and then dropped down to within one layer thickness after the fresh liquid resin has flowed under the cured cross-sections. Next, the control computer directs the UV light source or the like to trace a pattern of a next cross-section at the interface at a sufficient exposure so that the cure depth of the cross-section will be sufficient to adhere to the cured cross-sections.
Traditionally, an SLA, after building a part, does not perform any of the other steps in the overall stereolithographic process illustrated in FIG. 2a. This overall process will now be described. In step 1, a solid or surface model of a part is designed on a CAD system. In step 2, the CAD model is oriented in CAD space to minimize problems downstream with the remaining stereolithographic steps, and also a base or support is added to attach to and support the part while it is being built. In step 3, the CAD model is formatted to provide a generic surface representation of the part called an .STL file which is compatible with an interface specification required to communicate the file to the SLA, and which interface enables the SLA to remain independent of the specific CAD system. In step 4, the .STL file for the part is "sliced" by a SLICE computer (with certain SLICE parameters such as layer thickness) to provide two dimensional cross-sectional data descriptive of successive cross sections of the part. Collectively, the cross-sectional data is referred to as an .SLI file. In addition, portions of the .STL file, representing the base or support for example, are optionally "sliced" separately to form their own .SLI files with different SLICE parameters from that used to slice other portions of the file. In step 5, the .SLI files are transmitted from the SLICE computer to another computer known as the PROCESS computer via ETHERNET or floppy disk. In step 6, all the .SLI files for a part are merged into a single file. In step 7, the user specifies certain part building parameters such as cure depth, exposure times, and laser delay and stepping values, as required by the part's geometry and end use. The laser beam does not move over the resin surface continually, but instead, moves in steps over the surface with a delay at each step. The laser step size and delay largely determines the exposure and hence cure depth achieved. In step 8, which is the part building step, the UV laser beam is directed by the PROCESS computer to trace out patterns represented by the two-dimensional cross-sectional data with the building parameters specified in step 7, with the result that the liquid resin is cured where the laser strikes to form successive layers of the part at the surface. The first layer adheres to a horizontal platform located just below the surface of the liquid resin. The platform is connected to an elevator which lowers the platform also under control of the PROCESS computer. After a layer is cured, the platform is caused to dip into the liquid so that fresh liquid will flow over the cured layer to provide a thin film from which the next layer will be cured. The next layer is drawn after a pause to allow the liquid surface to level. The exposure of the UV laser and the thickness of the fresh liquid are controlled so that the cure depth of the next layer is such that it will adhere to the cured layer. The process is repeated until all layers have been cured to form a reproduction of the three-dimensional part. In step 9, the part is drained to remove excess resin (hereinafter referred to as part cleaning), the part is then ultraviolet cured to complete the photopolymerzation process (hereinafter referred to as post-curing), and the supports are next removed. Optional finishing steps such as sanding, or assembly into working models and painting, may also be performed.
FIG. 2b illustrates the major components of an apparatus known as SLA-1 used to perform the steps described above. A more recent commercial embodiment is the SLA-250, which is functionally similar to the SLA-1, except for the addition of a leveling blade for leveling the surface of the higher viscosity resins typically used with the SLA-250. An even more recent embodiment is the SLA-500 which has a larger vat for building larger parts.
As illustrated, the major components comprise SLA 34 and post-curing apparatus 35 (hereinafter referred to as a "PCA"). The major subcomponents of the SLA are illustrated in FIG. 2c. As illustrated, the SLA comprises SLICE computer assembly 36, which is electrically coupled via ETHERNET to electronic cabinet assembly 37, which is also mechanically and electrically coupled to chamber assembly 38, and to optics assembly 39. As illustrated, the SLICE computer assembly comprises monitor 40, SLICE computer 41, and keyboard 42, integrated together as shown. The SLICE computer also interfaces to a CAD/CAM system (not shown) either through ETHERNET, floppy disks, or any other data transfer method, for the transfer of the .STL files.
The electronic cabinet assembly is illustrated both in FIGS. 2c and 2e, in which like components and subcomponents are identified with like reference numerals. As illustrated, the electronic cabinet assembly is coupled to the SLICE computer assembly by means of ETHERNET cable 43. Moreover, the electronic cabinet assembly, in turn, comprises PROCESS computer 44, keyboard 45, and monitor 46, integrated together as shown in FIG. 2c. In addition, as illustrated in FIG. 2e, the assembly further comprises laser power supply 47, and other power supplies (not shown) for the high-speed dynamic mirrors 49, and for the elevator motor 50. The assembly further comprises AC power distribution panel 48, a control panel (shown but not identified with a reference numeral), plug-in circuit boards for control of the monitor, keyboard, and optional printer (not shown), plug-in driver (not shown) for control of the high-speed dynamic mirrors, and plug-in driver (not shown) for control of the vertical (Z-stage) elevator. The control panel includes a power on switch/indicator, a chamber light switch/indicator, a laser on indicator, and a shutter open indicator.
Operation and maintenance parameters, including fault diagnostics and laser performance information, are displayed on the monitor. The operation of the PROCESS computer is controlled by keyboard entries. Also, work surfaces around the keyboard and disc drive are covered with FORMICA for efficient cleaning and long wear.
The optics assembly is illustrated in FIG. 2c, and comprises laser 51, laser cover 52, shutters 53, beam turning mirrors 54a and 54b, beam expander 55, dynamic mirrors 56, and optics cover 57. As illustrated, the laser and related optical components are mounted on top of the electronic cabinet and chamber assembly. Also, the laser and optics are covered with laser and optics covers 52 and 57, respectively, which may be removed to service the laser and optics. For safety reasons, the covers are attached to the chamber and electronic cabinet assembly with fasteners, and a special tool is required to unlock the cover fasteners. In addition, interlock switches are activated when the covers are removed. The interlock switches activate dual solenoid-controlled shutters to block the laser beam when either cover is removed.
The laser is preferably a helium cadmium (HeCd) laser. In addition, the shutters, beam turning mirrors, beam expander, and dynamic mirrors are all mounted on an optics plate (not shown) placed on top of the electronic cabinet and chamber assembly. As illustrated, the shutters are preferably rotary solenoid-activated shutters which are situated at the laser output to block the laser beam when a safety interlock is opened. The beam-turning mirrors reflect the laser beam along an optical path through the beam expander, which enlarges and then focuses the laser beam so it will achieve a certain size on the surface of the liquid resin. The dynamic mirrors are under control of the PROCESS computer, and direct the laser beam to trace out patterns on the surface of the liquid resin according to the cross-sectional data provided by the SLICE computer. A quartz window (not shown) separates the optics assembly from the chamber assembly, in which is placed in the liquid resin. The dynamic mirrors direct the laser beam along an optical path through the quartz window to the liquid resin surface. The quartz window allows the laser beam to enter the chamber assembly, but otherwise isolates the two assemblies.
The chamber assembly is illustrated in FIGS. 2c and 2d. As illustrated in FIG. 2c, the chamber assembly 38 comprises a chamber (not identified with a reference numeral), chamber door 57 which opens into the chamber, storage compartment 58, and storage compartment door 59. Turning to FIG. 2d, the chamber assembly further comprises platform 60, reaction vat 61, elevator 62, a first beam profiler 63, a second beam profiler (not shown) diagonally located across the vat from the first beam profiler, heater/fan 64, air filter 65, chamber light 66, and shelf 67.
The chamber in which the part is formed is designed for operator safety and to ensure uniform operating conditions. The chamber may be heated to approximately 40 degrees C. (104 degrees F.) by means of the heater/fan, and the air in the chamber is circulated and filtered by means of the heater/fan and air filter. The overhead light illuminates the reaction vat and work surfaces. An interlock on the cabinet door, preferably a UV blocking plexiglass access door, when opened, activates a shutter to block the laser beam.
The reaction vat is installed in the chamber on guides which align it with the elevator and platform. The liquid resin is then placed in the vat.
The platform is attached to the vertical axis (or Z-stage) elevator. A part is formed cross-section by cross-section on the platform which is successively immersed in the liquid resin and therefore lowered into the vat while the part is being formed. After a part is formed, the platform on which the part is placed is raised to a position above the vat, and then disconnected from the elevator and removed from the chamber for post processing. Handling trays are provided to catch dripping resin.
The beam profilers are mounted diagonally across the reaction vat from one another at the focal length of the laser. The dynamic mirrors are periodically directed to direct the laser beam onto the beam profilers, which measures the beam intensity profile. The data may be displayed on the terminal either as a profile with intensity contour lines or as a single number representing the overall (integrated) beam intensity. This data is used to determine whether the mirrors should be cleaned and aligned, whether the laser should be serviced, and what building parameter values such as laser stepping and delay values (which together largely determine exposure) will yield cured resin of a particular cure depth and width.
The PCA is illustrated in FIG. 2f. As illustrated, the PCA comprises a chamber (not identified with a reference numeral), UV lamps 68 on adjustable stems, turntable 69, front and top doors 70a and 70b, respectively, with respective front and top UV blocking windows 73a and 73b, cooling and vent fan 71, control panel 72 with power switch and timer (not shown), and stand 74.
The UV lamps are preferably three 400 watt metal-halide UV-enhanced lamps, with reflectors, which can be positioned in the chamber for optimal post curing. The turntable is preferably a one revolution per minute turntable which rotates the part for uniform post-curing. The doors, located at the front and top, are for loading and unloading parts. Both doors are interlocked to turn off the UV lamps and turntable when they are opened, and have UV blocking windows to block the passage of UV light to allow safe viewing of the parts. The cooling and vent fan is preferably a 240 cubic feet per minute fan.
Turning again to FIG. 2a, as indicated, after a part is built, it is further processed in a post-processing step. A typical post-processing step is comprised of the following substeps:
1) Raising the part out of the vat of resin. PA1 2) Allowing the part to drain into the vat (e.g. from about 10 minutes to about an hour). PA1 3) Removing the part (and platform, if desired) from the SLA and placing it on an absorbent pad. PA1 4) Optionally placing the part/platform in a low temperature oven (heated to a temperature between room temperature and a temperature effective for thermally curing the resin, e.g., from about room temperature to about 100 degrees C., and preferably from about 60 to about 90 degrees C.). PA1 5) Optionally removing excess resin with cotton swabs. PA1 6) Optionally coating the part surface with resin to give good surface finish. PA1 7) Optionally giving the part a quick exposure of flood UV (or other radiation as appropriate for the photoinitiator in the resin) to set the surface. PA1 8) Optionally immersing the part in cool water, such that the water fills all cavities in the part, if possible. PA1 9) Applying flood UV light to the part (or other radiation as appropriate for the photoinitiator in the resin), optionally while the part is under water (or another appropriate liquid medium). PA1 10) Optionally rotating the part as necessary to provide a uniform cure. PA1 11) Removing the part from the platform, if necessary. PA1 12) Repeating substeps 6-10, if necessary.
As set forth in more detail in co-pending U.S. patent application Ser. No. 268,408, in carrying out substeps 3) and 4), once the part is placed on the absorbent pad, it can be left to drain in the open air at room temperature for a period of time ranging anywhere from minutes to a couple of days, after which time it can be placed into an oven or otherwise heated. In addition, the following, sometimes competing, considerations should be balanced when carrying out these substeps. It is important to avoid part warpage due to temperature, gravity, or other conditions capable of exerting a force on the part; to remove all excess resin from the surfaces of the part before further post-processing; and to avoid over absorption of oxygen, which inhibits post curing (substeps 8), 9) and 10) above) since oxygen acts as an inhibitor of the chemical reaction involved in later photopolymerization.
Optionally, the use of solvents can be combined with the use of the absorbent pad and oven heating to drain the part. In this approach, the part is contacted, e.g. by spraying, brushing, soaking, or the like, with a solvent suitable for reducing the viscosity of the resin on the surface of the part, without leaching unpolymerized resin from inside the part. The liquid surface resin and the solvent form a solution of relatively lower viscosity, causing the resin to drain more quickly from the part. Suitable solvents depend on the resin, but are, in general, organic solvents, preferably of intermediate polarity, such as methanol, methlylethylketone, ethanol, isopropyl alcohol, trichloroethane, or the like.
As set forth in more detail in co-pending U.S. patent application Ser. No. 268,428, in carrying out substeps 6) and 7), surface discontinuities in the part may then be filled in with resin to provide a smooth surface finish. Surface discontinuities may arise, for example, because of the stepwise nature of part building, which may leave the part with a stairstep appearance and corresponding rough surface finish. The part may then be given a rapid exposure, e.g. from a few seconds to no more than about a minute, preferably about 15 to about 30 seconds, of flood UV light (or other radiation appropriate to the resin being used) to set the surface. The process of coating and setting with UV light can be repeated several times, as necessary, to obtain a smooth surface finish over the entire part. In addition, the UV light should strike the surface of the part as uniformly as possible, which can be accomplished, for example, by using multiple lamps simultaneously from different angles, by using high quality reflectors to distribute the light around the part, by rotating the part or the lamps, or the like. As described in more detail in the next section, the UV lamps are preferably mercury discharge or metal halide lamps, such as, for example, PHILLIPS' (Belgium) HPA 400S "MERCURY VAPOR BURNER."
As explained in co-pending U.S. patent application Ser. No. 182,830, and its co-pending continuation-in-part Ser. No. 331,644, sometimes the part is built deliberately undersized so that when the surface discontinuities caused by the stairstep appearance of the part are filled in, the part will be the right size. Alternatively, as explained in those applications, sometimes the part is built deliberately oversized so that after sanding to remove the stairstep appearance, the part will be the right size. Another alternative is to build a part as close as possible to the correct size. In these instances, substeps 6) and 7) are eliminated.
As set forth in more detail in co-pending U.S. patent application Ser. No. which is now U.S. Pat. No. 4,996,010; and its co-pending continuation-in-part, Ser. No. 268,429, in carrying out substep 8), the part is then immersed in water, or another liquid medium such as a salt solution, a fluorocarbon such as trichlorotrifluoroethane, an organic solvent, such as polyethylene glycol telomers or ethanol, etc. during a flood UV exposure (or exposure to other radiation depending on the resin) of sufficient duration to completely cure the part. This substep may be necessary, since after the part building step, the part may not be completely cured, but instead may only be partially cured, and in a corresponding "green" state.
The liquid medium preferably has a similar specific gravity to the partially polymerized part, to provide an optimum level of buoyancy, such that distortion risks due to gravity are minimized; preferably absorbs infrared radiation coming from the UV light source so that the part is not heated by the infrared energy, and also absorbs heat away from the part that is generated during the polymerization reaction; and preferably acts as a heat exchanger by transmitting heat away from the part without a pronounced increase in the temperature of the liquid medium itself. Preferably, the liquid and part are at ambient temperature when the part is immersed in the liquid, e.g. the liquid is generally between about 15 and about 35 degrees C. Ideally, the liquid temperature is maintained at a selected level, typically within plus or minus 5 degrees C., as long as there is sufficient water to contribute a large enough thermal mass such that its temperature does not change dramatically during exposure to radiation from the UV lamps during the curing process. Optionally, the liquid can be recycled through a heat exchanger to maintain it substantially at room temperature and to reduce the quantity of water required. Another option is to filter the liquid to remove impurities, or the UV light may be filtered to remove certain peak absorption wavelengths from the impinging radiation to promote uniform curing by adding a suitable photoinitiator to the liquid to perform the same function.
Post-curing is performed using the PCA illustrated in FIG. 2f, which is described in the earlier section. The part is first placed in a vessel transparent to UV light, such as a quartz or PYREX container, and should then be placed inside the PCA and situated with respect to the UV light so that the UV light strikes the entire surface of the part as uniformly as possible.
As with substeps 6) and 7), this can be carried out by using multiple lamps simultaneously from different angles, using high quality reflectors to distribute the light around the part or by rotating the part or the lamps or the like.
The UV lamps are preferably high pressure mercury discharge or metal halide lamps, such as, for example, Phillips' (Belgium) HPA 400S "MERCURY VAPOR BURNER." The UV light is typically of mixed wavelengths in the about 250 to about 750 nm range, with the majority in the range of about 300 to about 400 nm. Moreover, the intensity of the light striking the surface is about 20 mW per square cm to about 100 mW per square cm in the about 300 to about 400 nm range. The post-cure time will typically range from about 7 to about 15 minutes.
A significant disadvantage of the above approach is that the post-processing substeps are manual, time-consuming, and subject to significant errors in operating conditions because of the absence of computer control of these substeps. In addition, these substeps are typically performed in physically separate areas, which require the part to be transported from area to area to complete the stereolithographic process, and also require a large physical area. The result is that the overall stereolithographic process is extremely fragmented.
In addition, regarding post-processing substeps 2)-5), the part cleaning substeps, additional problems are that it is difficult to completely remove excess resin, especially from difficult-to-reach areas of the part such as corners or detailed features using thermal draining, and it is also difficult to efficiently dispose of the excess resin. If a solvent is used, disposal of the solvent intermixed with resin is also a problem. Even if the part is allowed to drain on an absorbent pad, the absorbent pad must be disposed of after the part has drained excess resin into it. Disposal is an important environmental consideration, as the resin tends to be tacky and difficult to clean up, and certain resins may have a level of toxicity associated with them, so that disposal may be complicated from a health perspective. Moreover, the exposure of the part to oxygen may inhibit subsequent post-curing because of the action of oxygen in inhibiting polymerization. Also, the action of gravity while the part is draining may distort the part. In addition, the optional use of cotton swabs in substep 5) to clean hard-to-reach portions of the part is very time consuming, and also does not readily provide for uniform application from part to part. Finally, in substep 4), the low temperature oven, if allowed to act on the part too long, can lead to distortion and additional undesirable curing, especially if the resin is thermally-curable.
A disadvantage of the part-building step described above is that the inventory of resin required may substantial, particularly for the building of large parts. The large inventory of resin is required since the size of the vat in which is placed the liquid resin must be scaled to the size of the part, and the entire vat must be filled with resin even though a part may only require a quart of resin to build, and the resin which remains in the vat may have to be replaced about every six months or less, since this is the shelf-life of the resin. A large inventory may be required to replenish this turnover without significant delays. In addition, large parts may require a large vat, which may further increase the inventory requirements. In fact, the inventory required to support a large vat may be a reason why many commercial embodiments of SLA's are limited to building small parts. Besides the expense, the large inventory has the additional disadvantage that the pace of technological change in the development of resins is great, and a large inventory might become obsolete by the time it is used.
It is an object of the present invention to provide means with which to integrate the various stereolithographic steps or substeps described above together. Another object is to improve part-cleaning by making it more effective, by reducing the distortion which part cleaning may cause, and by reducing or eliminating the disposal problem. A further object is to reduce the resin inventory required for the building of larger parts.